During the development of an organism, most tissues achieve a predictable size during development. The mechanisms that regulate the growth, proliferation and survival of individual cells in a spatiotemporal manner to achieve the final size and shape of the organ are poorly understood. Our laboratory has used a genetic approach in the fruit fly Drosophila melanogaster to identify genes that regulate cell proliferation and organ size. Among the genes characterized in previous funding cycles include several components of the Hippo pathway, archipelago, which encodes a ubiquitin ligase that targets Cyclin E and Myc for degradation and the Tsc1 and Tsc2 genes. In addition to regulating growth during development, these genes are of interest because their human orthologs are mutated in several types of cancer. We have recently found that the spitfire gene, which encodes a ubiquitin ligase, functions as a negative regulator of tissue growth and regulates the levels of the protocadherin Fat at the cell surface. Furthermore Spitfire and Fat appear to co-localize at the apical membrane and in a population of vesicles. We propose to elucidate the mechanism by which Spitfire regulates Fat localization and function. This will be achieved by identifying substrates of Spitfire using approaches that utilize mass spectrometry as well as by conducting genetic screens aimed at identifying regulators and effectors of Spitfire. We will also test whether Spitfire regulates other signaling pathways. We have also found that differences in expression levels of the autophagy regulator Atg2 between neighboring cells can influence tissue growth and cell survival near the clonal boundary. We will determine whether this phenomenon is due to a role for autophagy in cell-competition or similar process or, alternatively, whether a subset of Atg genes has a role in regulating cell proliferation and survival via an autophagy-independent pathway.